A study is underway to determine the importance, the cellular identities and the mechanisms of the local control of hematopoiesis and hematopoietic cell dynamics by the stromal tissues and cells (often called the hematopoietic microenvironment) in which the hematopoietic cells are found. For purposes of study and, also, because evidence has come to light supporting such a division, the stromal controls have been divided as follows: 1) Lodgment of the hematopoietic stem cells in appropriate niches, 2) Commitment of the multipotent stem cell to a single line of differentiation, i.e., erythrocytic, granulocytic or megakaryocytic, 3) Proliferative stimulus to the stem cells and their immediate descendants to multiply. Tissue transplantation, tissue culture, E.M. studies and selective damage of hematopoietic stroma are used in an attempt to determine cellular identities and modes of action.